Method for fabricating closed vias in a printed circuit board

ABSTRACT

A method for forming closed vias in a multilayer printed circuit board. A dielectric layer is laminated to one side of a central core having a metal layer on each side. A second dielectric layer is laminated to the other side of the central core. Closed vias in the central core have been formed by drilling partially through but not completely penetrating the central core, and then completing the via from the opposite side with a hole that is much smaller in diameter to form a pathway that penetrates completely through the central core from one side to another. The via is then plated with metal to substantially close the smaller hole. Approximately one half of the closed vias are situated such that the closed aperture faces one dielectric layer and a remainder of the closed vias are situated such that the closed aperture faces the other dielectric layer. Resin from one dielectric layer fills the cavities of approximately one half of the closed vias, and resin from the other dielectric layer fills the circular cavities of the remainder of the closed vias. The total amount of resin migrated from each of the dielectric layers into the closed via cavities is approximately equal.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to an application entitled “PRINTED CIRCUITBOARD HAVING CLOSED VIAS”, filed on even date herewith, furtheridentified by attorney docket number CML03678AT, and assigned toassignee hereof.

FIELD OF THE INVENTION

The present invention relates generally to printed circuit boards. Moreparticularly, this invention relates to methods of forming blind vias inmultilayer printed circuit boards.

BACKGROUND

The continual drive to create smaller, lighter, and thinner portableelectronic devices, such as cellular telephones, requires thatcomponents, and especially the printed circuit board, must not limit theshape and size of these devices. The printed circuit board (PCB) mustoccupy minimal volume and accommodate the latest high density chip scalecomponent packages. Multilayer PCBs route signals from one layer toanother by means of vias, creating a compact, high density substrate.Currently, a designer does not have complete freedom to specify a layerX to layer Y connection, but is restricted to a few cost-effective viaoptions such as mechanical and laser drilling, sequential lamination,and build up. Mechanically drilled vias can penetrate the entire PCB,but they occupy space on every layer. Laser drilled blind microvias canbe employed to reduce the size and cost of the PCB, and, compared tomechanical drilling, laser drilling allows much smaller vias, and hencesmaller via capture pads. Unfortunately, in a build-up construction,laser formed blind vias can only connect the outermost layer to the nextinner layer. Additional size reduction could be achieved with solidlyplated microvias, which allows for via stacking and fan-out for highdensity chip scale component packages. But solid vias requirespecialized plating techniques, and are only possible for vias less than4 mils in diameter and in dielectrics less than 2 mils thick. Therefore,many board designs continue to use mechanical blind vias, foregoing thesize reduction advantages of laser microvias.

Additionally, buried vias (the plated holes in the interior layers of aPCB) provide z-axis interconnection, and do not consume real estate onthe outer layers of the PCB. Buried vias have often been combined withlaser drilled microvias, but it is difficult to obtain equal and uniformdielectric thickness on either side of the central core when the outerlayers are laminated to the core. This is because resin from the variouslayers does not fill the buried vias in a predictable, balanced, oruniform manner. Resin flow can vary so much that the resin thickness onthe top and bottom layers can differ by as much as 2 mils, which isunacceptable for circuits that have critical impedance requirements. Inaddition, inconsistent resin flow can cause dimples and voids, which arealso unacceptable.

It is therefore highly desirable to find a means of creating highdensity microvias in multi-layer PCBs for use in radio frequency (RF)applications and other circuits that have critical impedancerequirements.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures, where like reference numerals refer toidentical or functionally similar elements throughout the separate viewsand which together with the detailed description below are incorporatedin and form part of the specification, serve to further illustratevarious embodiments and to explain various principles and advantages allin accordance with the present invention.

FIG. 1 is a cross-sectional view of a hole drilled partially through aprinted circuit board in accordance with certain embodiments of thepresent invention.

FIG. 2 is a cross-sectional view of the printed circuit board of FIG. 1,depicting a laser beam piercing completely through the remainingthickness portion of the printed circuit board to the partially drilledhole, in accordance with certain embodiments of the present invention.

FIG. 3 is a cross-sectional view of the printed circuit board of FIG. 2,after metal is plated on the walls of the hole to close the laserpierced hole, in accordance with certain embodiments of the presentinvention.

FIG. 4 is a cross-sectional view of a printed circuit board having metalfoil selectively removed from two sides in accordance with certainembodiments of the present invention.

FIG. 5 is a cross-sectional view of the printed circuit board of FIG. 4,depicting a laser beam selectively removing dielectric material, inaccordance with certain embodiments of the present invention.

FIG. 6 is a cross-sectional view of a printed circuit board having metalfoil and dielectric material selectively removed from one side inaccordance with certain embodiments of the present invention.

FIG. 7 is a cross-sectional view of the printed circuit board of FIG. 6,depicting a laser beam selectively removing dielectric material, inaccordance with certain embodiments of the present invention.

FIG. 8 is a cross-sectional view of the printed circuit board of FIG. 7,depicting a laser beam piercing through metal foil on one side, inaccordance with certain embodiments of the present invention.

FIG. 9 is a cross-sectional view of the printed circuit board of eitherFIG. 5 or FIG. 8, after metal is plated on the walls of the hole toclose the laser pierced metal foil, in accordance with certainembodiments of the present invention.

FIG. 10 is an example of a fully laminated printed circuit board, inaccordance with some embodiments of the invention.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions of some of the elements inthe figures may be exaggerated relative to other elements to help toimprove understanding of embodiments of the present invention.

DETAILED DESCRIPTION

Before describing in detail embodiments that are in accordance with thepresent invention, it should be observed that the embodiments resideprimarily in combinations of method steps or actions and apparatuscomponents related to closed vias in a multilayer printed circuit board.Accordingly, the apparatus components and method steps have beenrepresented where appropriate by conventional symbols in the drawings,showing only those specific details that are pertinent to understandingthe embodiments of the present invention so as not to obscure thedisclosure with details that will be readily apparent to those ofordinary skill in the art having the benefit of the description herein.

In this document, relational terms such as first and second, top andbottom, and the like may be used solely to distinguish one entity oraction from another entity or action without necessarily requiring orimplying any actual such relationship or order between such entities oractions. The terms “comprises,” “comprising,” or any other variationthereof, are intended to cover a non-exclusive inclusion, such that aprocess, method, article, or apparatus that comprises a list of elementsdoes not include only those elements but may include other elements notexpressly listed or inherent to such process, method, article, orapparatus. An element preceded by “comprises . . . a” does not, withoutmore constraints, preclude the existence of additional identicalelements in the process, method, article, or apparatus that comprisesthe element.

It will be appreciated that embodiments of the invention describedherein may be comprised of one or more conventional processes and/ormaterials. Of course, a combination of processes and/or materials couldbe used. Thus, methods and means for these functions have been describedherein. Further, it is expected that one of ordinary skill,notwithstanding possibly significant effort and many design choicesmotivated by, for example, available time, current technology, andeconomic considerations, when guided by the concepts and principlesdisclosed herein will be readily capable of creating a high densitymultilayer printed circuit board with minimal experimentation.

A printed circuit board substrate comprises N layers of dielectriclaminate and N+1 layers of metal, for example a single layer ofdielectric laminate with a layer of metal on each side, or three layersof dielectric laminate with four layers of metal. An N-layer circuitboard is fabricated by laminating a first metal foil and a firstdielectric prepreg layer to a first side of an N−2 layer printed circuitboard core having N−1 layers of metal, and simultaneously laminating asecond metal foil and a second dielectric prepreg layer to a second sideof the core. Prior to this lamination, closed vias in the central coreare formed by drilling partially through but not completely penetratingthe central core, and then completing the via from the opposite sidewith a hole that is much smaller in diameter to form a pathway thatpenetrates completely through the central core from one side to another.The via is then plated with copper to substantially close the smallerhole. Approximately one half of the closed vias are situated such thatthe closed aperture faces one dielectric prepreg layer and a remainderof the closed vias are situated such that the closed aperture faces theother dielectric prepreg layer. Resin from one dielectric prepreg layerfills the circular cavities of approximately one half of the closedvias, and resin from the other dielectric prepreg layer fills thecavities of the remainder of the closed vias. The total amount of resinmigrated from each of the dielectric prepreg layers into the closed viacavities is approximately equal.

Referring now to FIG. 1, a closed microvia is formed in a printedcircuit board in a first embodiment by starting with a central core 100comprising a dielectric laminate 110, typically a resin such as epoxy orpolyimide that is reinforced with chopped or woven glass fibers.Laminated to each side of the dielectric laminate 110 is a thin layer ofmetal foil 120, 130 such as copper. While FIG. 1 illustrates thesimplest case of a single dielectric laminate and two metal layers, morecomplex printed circuit cores are also possible, for example a core withthree dielectric layers and four metal layers, or any combination of Ndielectric layers and N+1 metal layers. An aperture 140 is formed in thePCB by mechanically drilling, with for example, a carbide drill 145,completely through the metal foil 120 on one side of the core 110 andpartially into, but not completely through, the core 110. In analternate embodiment, this blind hole can be formed with a laser. Thedrilling is carefully controlled so as to stop prior to piercing themetal foil 130 on the opposite side of the core 110. Ideally, thedrilled hole would stop exactly at the inner surface of the second layerof metal foil 130, but in practice, this may not always be achievable,due to mechanical tolerances of the process equipment. The upper layerof metal foil 130 might be dimpled or partially pierced during thedrilling, but any opening in the metal foil should be minimal. If thedrilled hole stops short of the inner surface of the metal foil, thensome dielectric material will remain at the bottom of the hole, and willbe subsequently removed next. Referring now to FIG. 2, a laser beam 160is impinged upon the side of the PCB opposite to the side that the holewas drilled from, to penetrate the metal foil and remove any dielectricmaterial that may remain at the bottom of the partially drilled hole.The laser beam forms a hole 150 in the metal foil 130 that issubstantially co-axial to the aperture 140, the diameter of the laserformed hole being less than 75%, and preferably less than 50%, of thediameter of the mechanically drilled hole. The large mechanicallydrilled hole 140 and the small laser formed hole 150 combine to create acomplete pathway that penetrates through the printed circuit laminatefrom one side to another. Of course, one skilled in the art will realizethat a plurality of these combined holes is formed in the typical PCB,at various locations as dictated by the PCB layout designer.

Referring now to FIG. 3, after the through-via is formed in the laminatewith two different sized openings, the laminate is then plated withmetal plating, typically copper, in one or more of a number ofconventional manners, such as electroless or electrolytic plating, forexample, or a combination thereof. Copper 320, 330 is plated onto thesurfaces of the metal foils 120, 130 on both sides of the laminate, andcopper 340, 350 is also plated on the walls of the mechanically drilledaperture 140 and on the walls of the laser formed aperture 150,respectively. The diameter of the laser formed aperture 150 and thethickness of the plating 350, 330 are chosen such that the plating inthe laser formed aperture substantially fills it to form a closedmicrovia. That is, as a general rule, the plating thickness should beapproximately one half of the diameter of the laser formed hole. Theintent is to close the small hole, and to plate the walls of the largehole, so that an electrically continuous path is created from one metallayer 120 on one side of the laminate to the other metal layer 130 onthe other side of the laminate.

Having described a simple process for fabricating a closed microvia or“bottleneck via,” (a plated-through-hole where one end is capped using asingle plating step and a combination of mechanical controlled depthdrilling and coaxial laser boring), we now describe a further embodimentdepicted in FIG. 10. A first metal foil 1092 and a first dielectricprepreg layer 1085 are laminated to the metal foil 120 on one side ofthe laminate 100, and a second metal foil layer 1090 and a seconddielectric prepreg layer 1080 are laminated to the metal foil 130 on theother side of the laminate. A plurality of the closed microviascomprising the two different sized co-axial apertures described aboveare distributed throughout the laminate such that approximately one halfof the closed microvias are arranged to have the small, closed apertures355 facing the first dielectric prepreg layer and a remainder of theclosed vias are situated to have the small, closed apertures 350 facingthe second dielectric prepreg layer. When the three layers are bondedtogether with heat and pressure, the resin 1085 from the firstdielectric prepreg layer flows to fill the large cavities of theapproximately one half of the closed vias, and resin 1080 from thesecond dielectric prepreg layer flows to fill the large cavities of theremainder of the plurality of closed vias, so that the total amount ofresin that might migrate from each of the dielectric prepreg layers intothe respective closed microvias is approximately equal. In this way, theresulting thicknesses of the two outer dielectric layers in their final,cured state are equal, or nearly so, because of the balanced flow ofresin. Moreover, by judicious arrangement of the direction of the closedends 350, 355 of the microvias, the resin flow in the multilayer PCB canbe ‘balanced’ locally as well as globally so that no local crosssectional dimension is less than 90% of the maximum thickness of themultilayer PCB.

Referring now to FIG. 4, another embodiment of our invention forms aclosed microvia by first selectively removing portions of metal tocreate substantially co-axial openings 450, 440 in metal foil layers120, 130 that are clad onto an insulating dielectric core 110. Theinsulating dielectric core 110 is typically a resin such as epoxy orpolyimide that is reinforced with chopped or woven glass fibers.Laminated to each side of the core 110 is a thin layer of metal foil120, 130 such as copper. The co-axial openings 440, 450 are typicallycircular in shape, but may be other shapes, and are mutuallysubstantially co-axial and substantially co-axial to a closed microviato be formed subsequently in the following actions. Selectively removingthe metal exposes the surface 445, 455 of the underlying dielectric, andis accomplished in conventional fashion, such as chemical etching,mechanical milling, or laser ablation. The two openings 440, 450 aresituated so that one opening is substantially co-axial to the otheropening, or as close as possible, given state-of-the-art tolerances. Oneopening 450 is substantially smaller than the other opening 440, withthe diameter of the smaller opening being less than 50-75% of thediameter of the corresponding larger opening. As shown by the dashedlines in FIG. 4, a portion 470 of the dielectric that underlies thelarger opening 440 will subsequently be removed next (FIG. 5). The metalfoil 130 that remains on the surface of the dielectric core 110 acts asa mask for a laser ablation step. A laser beam 560 is impinged on thelarge opening 440 in order to ablate or vaporize away the dielectricmaterial 470, leaving a void 570. The parameters of the laser beam arechosen so as to vaporize the dielectric material 470, but not to ablateor vaporize the copper foils 120, 130, as known to those of ordinaryskill in the art. For example, a carbon dioxide laser is known toprovide high selectivity in ablating polymer resins and woven glass withrespect to copper foil. A passage has now been formed that penetratescompletely through the dielectric core 100. Referring now to FIG. 9, thetreated structure is then plated with metal, typically copper, inconventional fashion, such as electroless or electrolytic plating or acombination thereof, to coat the walls 940 of the via and the surfaces920 of the metal foils 120, 130. The copper plating 950 in the smallopening 450, 850 effectively closes the opening. The diameter of thesmaller opening and the thickness of the plating are chosen such thatthe plating in the small opening substantially closes it to form aclosed microvia. That is, as a general rule, the plating thicknessshould be approximately one half of the diameter of the small opening.The intent is to close the small hole, and to plate the walls of thelarge opening and the laser etched hole in the dielectric core, so thatan electrically continuous path is created from one metal layer 120 onone side of the laminate to the other metal layer 130 on the other sideof the laminate. As described previously, additional prepreg layers cannow be laminated onto each side of the microvia-containing laminate toform a multilayer PCB. A plurality of these closed microvias can bedistributed throughout the laminate such that approximately one half ofthe closed microvias are arranged to have the small, closed apertures950 facing in one direction and a remainder of the closed vias can besituated to have the small, closed apertures facing in an oppositedirection. By judiciously selecting the direction that the closed ends950 of the microvias face, the resin flow in the multilayer PCB can be‘balanced’ so that no local cross sectional dimension is less than 90%of the maximum thickness of the multilayer PCB. Moreover, the closedends of the microvias permit additional microvias in the next layer oflaminate to be “stacked” directly on top of these closed ends. Theinvention thus enables an “any layer via” multilayer board constructionin which laser microvias are used for all vertical interconnect.Mechanically drilled vias and “staggered” microvias can both beeliminated, providing significant space savings.

Referring now to FIG. 6, still another embodiment of our invention formsa closed microvia by first selectively removing portions of metal inconventional fashion, such as chemical etching, mechanical milling,carbon dioxide laser ablation, or ultraviolet laser ablation 660 tocreate an opening 640 in only one metal foil layer 130 that is clad ontoan insulating dielectric laminate 110. The opening 640 is typicallycircular in shape, but may be other shapes, and corresponds to alocation of a closed microvia to be formed in subsequent actions. Whileit is desirable to remove solely metal, sometimes a small portion of theunderlying dielectric is removed, and this is acceptable. The metal foil130 that remains on the surface of the dielectric laminate 110 acts as amask for a second laser ablation step (FIG. 7). A laser beam 760 isimpinged on the large opening 640 in order to ablate or vaporize awaythe dielectric material, leaving a void 770. The parameters of the laserbeam are chosen so as to vaporize the portions of the dielectricmaterial 110, but not to etch or vaporize either the copper foil 130 onthe upper side or the metal foil 120 on the lower side of the laminate.At this point, a blind hole 770 has been formed that penetrates throughthe upper layer of metal foil 130 and the dielectric material 110 in thecore, but not through the lower layer of metal foil 120. Referring nowto FIG. 8, a carbon dioxide or ultraviolet laser beam 660 is impinged inthe hole 770 to form an opening 850 in the lower layer of copper foil120. This can also be performed by impinging the laser beam on theopposite side of the PCB. The two openings 640, 850 are situated so thatone opening is substantially co-axial to the other opening and to acentral axis of the void 770 in the dielectric layer, or as close aspossible, given state-of-the-art tolerances. The opening 850 in thelower layer of foil 120 is substantially smaller than the opening 640 inthe upper layer of foil 130, with the diameter of the smaller openingbeing less than 50-75% of the diameter of the corresponding largeropening. A passage has now been formed that penetrates completelythrough the structure. Referring now to FIG. 9, the treated structure isthen plated with metal, typically copper, in conventional fashion, suchas electroless or electrolytic plating or a combination thereof, to coatthe walls 940 of the opening 770 in the dielectric and the surfaces 920of the metal foils 120, 130. The copper plating 950 in the small opening850 effectively closes the opening. The diameter of the smaller openingand the thickness of the plating are chosen such that the plating in thesmall opening substantially closes it to form a closed microvia. Thatis, as a general rule, the plating thickness should be approximately onehalf of the diameter of the small opening. The intent is to close thesmall hole, and to plate the walls of the large opening in the metalfoil and the laser ablated hole in the dielectric core, so that anelectrically continuous path is created from one metal layer 120 on oneside of the laminate to the other metal layer 130 on the other side ofthe laminate. As described previously, additional prepreg layers can nowbe laminated onto each side of the microvia-containing laminate to forma multilayer PCB. A plurality of these closed microvias can bedistributed throughout the laminate such that approximately one half ofthe closed microvias are arranged to have the small, closed apertures950 facing in one direction and a remainder of the closed vias arearranged to have the small, closed apertures facing in an oppositedirection. By judiciously selecting the direction that the closed ends950 of the microvias face, the resin flow in the multilayer PCB can be‘balanced’ so that no local cross sectional dimension is less than 90%of the maximum thickness of the multilayer PCB.

In the foregoing specification, specific embodiments of the presentinvention have been described. However, one of ordinary skill in the artappreciates that various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. Accordingly, the specification and figures are to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope ofpresent invention. The benefits, advantages, solutions to problems, andany element(s) that may cause any benefit, advantage, or solution tooccur or become more pronounced are not to be construed as critical,required, or essential features or elements of any or all the claims.The invention is defined solely by the appended claims including anyamendments made during the pendency of this application and allequivalents of those claims as issued.

1. A method of manufacturing a closed via in a printed circuit board,comprising: providing a printed circuit laminate having metal foil onboth sides; providing a first opening in the metal foil on one side ofthe printed circuit laminate; providing a second opening in the metalfoil on the opposing side of the laminate substantially co-axial to thefirst opening, a diameter of the second opening being less than 75% of adiameter of the first opening; removing dielectric material in theportion of the printed circuit laminate exposed by the first opening toform a pathway completely through the laminate from one side to another;and providing metal plating on walls of the pathway and in the secondopening at a thickness sufficient to substantially close the secondopening to form a closed via.
 2. The method as described in claim 1,wherein a plurality of closed vias are formed in the printed circuitboard.
 3. The method as described in claim 2, wherein the plurality ofclosed vias is arranged such that the closed second opening ofapproximately one half of the plurality of closed vias faces one side ofthe printed circuit laminate and the closed second opening of aremainder of the plurality of closed vias faces the opposite direction.4. The method as described in claim 2, wherein the arranged plurality ofclosed vias is balanced so that no local cross sectional dimension ofthe printed circuit board is less than 90% of the maximum thickness ofthe printed circuit board.
 5. The method as described in claim 1,wherein the first and second openings are formed by pattern etching. 6.The method as described in claim 1, wherein the dielectric material isremoved by laser ablation.
 7. A method of manufacturing a closed via ina printed circuit board, comprising: providing a printed circuitlaminate having metal foil on both sides; providing a first opening inthe metal foil on a first side of the printed circuit laminate; removingdielectric material in the portion of the printed circuit laminateexposed by the first opening to expose an inner surface of the metalfoil on a second side of the printed circuit laminate; providing asecond opening in the metal foil on the second side of the laminatesubstantially co-axial to the first opening, a diameter of the secondopening being less than 75% of a diameter of the first opening so as toform a pathway completely through the laminate from one side to another;and providing metal plating on walls of the pathway and in the secondopening of a thickness sufficient to substantially close the secondopening on the second side of the laminate to form a closed via.
 8. Themethod as described in claim 7, wherein a plurality of closed vias areformed in the printed circuit board.
 9. The method as described in claim8, wherein the plurality of closed vias are arranged such that theclosed second opening of approximately one half of the plurality ofclosed vias faces one side of a printed circuit laminate and the closedsecond opening of a remainder of the plurality of closed vias faces theopposite direction.
 10. The method as described in claim 9, wherein thearranged plurality of closed vias is balanced so that no local crosssectional dimension of the printed circuit board is less than 90% of themaximum thickness of the printed circuit board.
 11. The method asdescribed in claim 7, wherein the first and second openings are formedby an ultraviolet laser or by a carbon dioxide laser.
 12. The method asdescribed in claim 7, wherein the dielectric material is removed bylaser ablation.
 13. A method of manufacturing a closed via in a printedcircuit board, comprising: providing a printed circuit core comprisingone or more layers of dielectric laminate interleaved with two or morelayers of metal foil; drilling partially through the printed circuitcore to form a cavity that does not penetrate the metal foil on anopposing side of the printed circuit laminate; providing an opening onthe opposing side of the laminate substantially co-axial to the cavity,a diameter of the opening being less than 75% of a diameter of thecavity so as to form a pathway completely through the laminate from oneside to another; and providing metal plating on walls of the pathway ofa thickness sufficient to substantially close the opening on theopposing side of the laminate to form a closed via.
 14. The method asdescribed in claim 13, wherein a plurality of closed vias are formed inthe printed circuit board.
 15. The method as described in claim 14,wherein the plurality of closed vias are arranged such that the closedopening of approximately one half of the plurality of closed vias facesone side of a printed circuit laminate and the closed opening of theremainder of the plurality of closed vias faces the opposite side. 16.The method as described in claim 15, wherein the arranged plurality ofclosed vias is balanced so that no local cross sectional dimension ofthe printed circuit board is less than 90% of the maximum thickness ofthe printed circuit board.